Liquid crystal display devices employ optical (refractive) anisotropy and dielectric anisotropy of liquid crystal substances, and are divided into various modes such as TN mode (twisted nematic mode), DS mode (dynamic scattering mode), G.H mode (guest-host mode), DAP (deformation of aligned phase mode), and STN mode (super twisted nematic mode) according to their display modes. Properties of liquid crystal substances suitable for each of the display mode are different. Recently, liquid crystal display devices particularly having a higher display quality are required, and demand for display devices of active matrix mode represented by TFT mode (thin film transistor mode) is increased in order to respond to the requirement. Liquid crystal substances used in display devices of any mode are necessary to be stable against moisture, air, heat, and light. Besides, the liquid crystal substances must exhibit a liquid crystal phase in a temperature range as large as possible with room temperature being its center, be low in viscosity, be excellent in miscibility with other liquid crystalline compounds (hereinafter used as a general term for the compounds having a liquid crystal phase and the compounds which do not impair a liquid crystal phase when mixed with other liquid crystal compounds) and liquid crystal compositions, have a high dielectric anisotropy value
(.DELTA..di-elect cons.), and have an optimum optical anisotropy value (.DELTA.n). However, liquid crystal substances which completely satisfy such requirements by single compound have not been found, and it is a current situation that liquid crystal compositions obtained by mixing several to several tens kind of liquid crystalline compounds are used.
It is lately required in particular to liquid crystal display devices of TFT mode that they maintain a high contrast during frame time and can favorably be driven even at low temperatures. In order to cope with these requirements, advent of liquid crystalline compounds and liquid crystal compositions exhibiting a higher voltage holding ratio than that of conventional liquid crystal materials is expected for the former, and advent of liquid crystalline compounds which more hardly deposit crystals or develop smectic phase at low temperatures, that is, liquid crystalline compounds excellent in the miscibility with other liquid crystalline compounds and liquid crystal compositions at low temperatures is hoped for the latter.
Heretofore, as liquid crystal materials having a high voltage holding ratio and a high .DELTA..di-elect cons., fluorine type compounds, for example, trifluorophenyl derivatives expressed by the formula (10) (Laid-open Japanese Patent Publication No. Hei 02-233626), and trifluoromethylphenyl derivatives expressed by the formula (11) and trifluoromethoxyphenyl derivatives expressed by the formula (12) (Laid-open Japanese Patent Publication No. Hei 04-506361) are known.
Whereas these compounds have an electron withdrawing group at one terminal, the alkyl chain length (total of the number of carbon atoms and the number of oxygen atom in the chain) in the group is 2 or less. Accordingly, these compounds readily deposit crystals or develop smectic phase at low temperatures when mixed with other liquid crystal substances. ##STR2##
On the other hand, as component of liquid crystal compositions which makes excellent driving of liquid crystal display devices of TFT mode possible even at low temperatures, compounds a part of which is replaced by a fluorine substituted alkoxy group expressed by the formula (13) in which the number of carbon atom in the electron withdrawing group at one terminal is 2 (Laid-open Japanese Patent Publication No. Hei 7-145099) and compounds expressed by one of the formulas (14) to (18) in which the number of carbon atom described above is 2 or more (WO 88/08441, DE 4142519, DE 4329592, DE 4308028, and DE 19513007 in turn) are also known.
These compounds having an electron withdrawing group at one terminal are still insufficient as component of liquid crystal compositions for driving at low temperatures, and for instance, are liable to cause an extreme lowering in response speed and poor display at low temperatures. ##STR3##
Further, compounds which are formed into a structure having an electron withdrawing group at both terminals by providing F, or an alkyl group or alkoxy group in which fluorine atom is introduced, for example, CF.sub.3 or OCF.sub.3 at the other terminal are known (Laid-open Japanese Patent Publication No. Hei 04-506817).
However, such compounds are not sufficient in the miscibility with other liquid crystals at low temperatures, and the compound expressed by the following formula (19) does not exhibit even a liquid crystal phase. ##STR4##
As described above, compounds excellent in the miscibility at low temperatures with other liquid crystals at low temperatures while having a high voltage holding ration and a high .DELTA..di-elect cons. have not been attained.